Refrigeration system

ABSTRACT

A refrigeration system having a plurality of cooled surfaces, each of which has a thermostatically controlled refrigerant supply valve. The valves are pulse-width modulated, each having the same period and being so controlled so they have staggered openings relative to one another thus providing a more uniform load on the system.

BACKGROUND OF THE INVENTION

The invention relates to a refrigeration system having a plurality ofcooling surfaces, each of which is connected by way of aseries-connected controlled valve to a common refrigerant supply device,and having a control arrangement connected to the valves.

The refrigerant supply device can here be formed by a heat-exchanger inwhich heat is extracted from brine. The heat is given up to a coolantwhich is cooled in a customary cooling circuit having one or morecompressors, a condenser and a collector, the heat exchanger beingprovided with an expansion valve. The brine then flows through thecooling surfaces and absorbs heat there in order to cool the environmentaround the cooling surfaces. In another practical form, the coolingsurfaces can also have refrigerant flowing directly through them, therefrigerant being channelled through a circuit containing one or morecompressors, a condenser and a collector.

A refrigeration system of the kind mentioned in the introduction isknown from EP 0 410 330 A2. The refrigeration system in that case has atleast two compressors arranged in parallel in the refrigerant circuit,which compressors can be operated jointly or alternately individually tosatisfy the cooling requirement at the various cooling pointssimultaneously. In this connection it is desirable for the frequency ofswitching-on of the individual compressors to be reduced in order toprolong their service life. A control arrangement which is connected tothermostatic valves is provided for controlling the compressors. Thethermostatic valves relay only the necessary temperature information tothe control unit, however, so that this controls the compressorsaccordingly. The control unit can also switch off individual coolingsurfaces when their actual value falls below a predetermined referencevalue.

SUMMARY OF THE INVENTION

The invention is based on the problem of rendering loading on therefrigerant supply device more uniform.

In a refrigeration system of the kind mentioned in the introduction,this problem is solved in that the control arrangement generates foreach valve a pulse-width modulated signal for operation of the valve,all signals having the same period, and in that the valves of theindividual cooling surfaces open at staggered intervals with respect toone another.

There is a more uniform call upon the output of the refrigeration supplydevice with this construction. Since the individual valves open atstaggered intervals, refrigerant is channelled through the coolingsurfaces also at correspondingly staggered intervals. The controlarrangement controls only the start of "refrigerant consumption",however. The end is determined for each cooling surface in dependence onits demand for refrigeration. The control arrangement controls the valveaccordingly, that is, it closes it when sufficient refrigerant hasflowed into the cooling surface. Viewed statistically, with asufficiently large number of cooling surfaces a state will then bereached in which always a few cooling surfaces are being supplied withrefrigerant, whilst other cooling surfaces are switched off. The largeris the number of cooling surfaces, the more uniform can one keep theloading on the refrigerant supply device. Useful results have alsoalready been obtained in practice when only three or four coolingsurfaces are operated in parallel with one another. Since all signalshave the same period, that is, all valves open similarly but atstaggered intervals, the refrigerating capacity can be distributed veryuniformly via the distribution of the refrigerant, so that the loadingon the refrigerant supply device is correspondingly uniform.

In a preferred construction, the period is constant. Not only are theperiods for the individual valves the same, but also successive periodsare the same. Control is therefore simplified. It is easier to definethe parallelism of the individual valves during operation in that theyare able to open at staggered intervals.

The valves are advantageously ON-OFF valves. Such valves can beoperated, for example, under clocked control. The ratio of the open timeto the sum of open and closed time then provides the opening degreewhich in turn determines the through-flow. At any rate, the valve isfully opened in the open time, so that is lets through the maximumpossible flow of refrigerant. In the closed time, on the other hand, thevalve is completely blocked. For that reason such a valve is especiallysuitable for the refrigeration system in question, because when it isopen, it introduces the maximum possible flow of refrigerant into thecooling surface, that is, it takes away refrigerant from the compressorsquickly but briefly, but in the closed state the refrigerant isavailable for other cooling surfaces.

In this connection, it is especially preferred for the controlarrangement to have for each valve a controller which determines a pulseduty factor for the valve in dependence on the demand for refrigerationand supplies a corresponding signal. The control is effected thereforeautonomously for each valve or for each cooling surface. The controllercan be in the form of a separate component which is housed in thevicinity of each cooling surface. Alternatively, it can be part of thecontrol arrangement. In particular, it can be realized in software or byprogrammed control. The controller receives temperature data from thecooling surface and adjusts the pulse duty factor so that this actualtemperature approximates as closely as possible to a predeterminedreference temperature. The reference value or reference temperature canbe entered in the controller in known manner.

The control arrangement preferably produces for each controller asynchronizing pulse which is staggered with respect to othersynchronizing pulses. Each controller responds identically to thesynchronizing pulse allocated to it. In the simplest case, on receivingthis synchronizing pulse, it opens the valve and holds it open for theproportion of the period which is needed based on the pulse duty factor.Alternatively, it is possible for the synchronizing pulse first toinitiate a computation algorithm, the length of which is the same forall controllers, that is, which has the same number of processing stepsfor all controllers, and by means of which the controller determines thepulse duty factor from the temperature difference between reference andactual temperature and then opens the valve. What matters here is merelythat all controllers operate identically, that is, open the valve alwayswith the same delay after the appearance of the synchronizing pulse.

In an alternative construction, provision can be made for the controlarrangement to generate a common synchronizing pulse for all controllersand for each controller to have a time-delay element, the delay time ofwhich differs from other delay times. Whereas a synchronizing pulse foreach controller requires each controller to be addressed or requires aseparate lead for each controller, with a common synchronizing pulse forall controllers a relatively simple transfer of data can be achieved.The time offset between the different valves is then achieved by thefact that the individual controllers respond to the synchronizing pulsewith different delays. The time offset can also be realized in this wayas the individual valves are triggered.

Each controller preferably has a timer which determines the period. Thistimer is admittedly required as an extra element for each controller,but in return there is no longer any need to transfer the periodadditionally from the control arrangement to the controllers. Thecontroller still requires the time information in order to determine thepulse duty factor.

In this connection it is especially preferred for the timers to beactivated by the synchronizing pulses only under predetermined operatingconditions. Such an operating condition can occur, for example, whenswitching on the refrigeration system. After that, the individualcontrollers can operate autonomously for a relatively long period oftime. Currently available timers are accurate enough to maintainparallel operation of the controllers also over a relatively long periodof time. Other operating conditions can be, for example, the change-overfrom day-time operation to night-time operation, when the refrigeratingcapacity is increased and reduced respectively. Finally, such asynchronizing pulse can be generated at predetermined longer periods oftime, for example, every eight hours.

Preferably, several cooling surfaces are combined in groups. When thereare many cooling surfaces, the delay time for an individual coolingsurface may possibly be too short, because the delay time equals theperiod divided by the number of cooling surfaces. If several coolingsurfaces are combined to form a group, the principle of shifted controlcan be transferred to the individual groups, that is, the coolingsurfaces of one group are supplied jointly, by triggering all valves ofthat group simultaneously, but the valves of different groups aretriggered at different times. The delay times become adequately longagain.

In parallel with the cooling surfaces there is preferably provided anoverflow path containing an overflow valve. Since it is possible for allcooling surfaces to be blocked by their valves, the overflow pathprovides an opportunity for refrigerant to circulate even whentemporarily there is no flow through the cooling surfaces. Such anarrangement is especially advantageous when a secondary circuit is beingused for the refrigerant, in which the refrigerant is formed, forexample, by brine. The overflow valve can optionally be replaced by asimple throttle.

It is also advantageous for the refrigerant supply device to have abuffered supply. This enables high and low load peaks to be catered for,without the compressors having to be reversed.

Each cooling surface is preferably additionally connected in series witha thermostatic expansion valve. The functions of the expansion valve andthe valve that is controlled by the control arrangement can therefore beisolated. The thermostatic expansion valve is chiefly responsible forfilling the cooling surface, whilst the other valve regulates the amountof coolant that is taken from the circuit to charge the cooling surface.

Additionally, an adjusting valve can also be provided in series with thecooling surface. Such an adjusting valve can, for example, limit theflow-through in cases in which dissimilar cooling surfaces are used inparallel with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinafter with reference to preferredembodiments in conjunction with the drawings, in which

FIG. 1 shows a refrigeration system operated with brine,

FIG. 2 shows a different embodiment of a refrigeration system,

FIG. 3 is a diagrammatic representation of signal waveforms in a firstembodiment and

FIG. 4 is a diagrammatic representation of signal waveforms in a secondembodiment.

DETAILED DESCRIPTION OF THE INVENTION

A refrigeration system 1 in FIG. 1 comprises a brine circuit 2. Here,the brine passes through the secondary side of a heat exchanger 3, thatis, an evaporator. The primary side of the heat exchanger 3 is suppliedfrom a refrigerant circuit 4. The refrigerant circuit 4 has a compressorgroup 5 comprising several compressors 5a, 5b, 5c, a condenser 6, acollector 7 and a thermostatic expansion valve 8, which is arrangedupstream of the heat exchanger 3. The compressor group 5 is controlledby a central unit 9 which is connected to a temperature sensor 10 whichdetermines the temperature of the brine in the brine circuit 2downstream of the heat exchanger 3. Depending on the temperature, thecompressors 5 are operated with a lower or higher output or individualcompressors are switched on or off.

Several cooling surfaces 11 are arranged in parallel with one another inthe brine circuit 2. Cooling surface shall be understood in thisapplication to mean any device which operates as heat exchanger betweena refrigerant or the brine and an ambient medium. This applies even whenthe cooling surfaces 11 are not of planar construction but have adifferent form.

Each cooling surface 11 is in series with a valve 12, which is connectedto a controller 13. From a temperature sensor 14 the controller 12receives information about the temperature of the cooling surface 11associated with the controller. Arranged in series with the coolingsurface 11 there is furthermore an adjusting valve 15, with which themaximum flow-through can be adjusted. This is especially advantageouswhen cooling surfaces of different sizes or having different flowresistances are used.

In parallel with the cooling surfaces 14 there is arranged an overflowvalve 16 in an overflow path 17.

All controllers 13 are connected to a common control unit. The controlunit 18 and the controllers 13 together form a control arrangement.

The central unit 9 can optionally also be part of the controlarrangement.

An alternative configuration, in which identical and corresponding partshave been given the same reference numbers, is illustrated in FIG. 2.The cooling surfaces 11 are here no longer supplied by a secondarycircuit which is fed with brine, but directly by the refrigerant fromthe refrigerant circuit 4. A thermostatic expansion valve 19 isaccordingly arranged upstream of each cooling surface 11, and receivesthe necessary temperature information by way of a temperature sensor 20.

The valves 12 are in the form of ON-OFF valves in both embodiments. Theytherefore have only two operating positions. In the ON position they arefully open and unblock a path for the refrigerant, which is then able toflow either directly or by way of the expansion valves 19 into thecooling surface 11. In the closed state the flow of refrigerant into thecooling surface 11 is blocked.

When the valves are controlled so that always a few valves 12 are openand other valves 12 are closed, a relatively uniform loading of therefrigerant supply device that is arranged in the refrigerant circuit 4can be achieved.

In order to embody this principle, the individual valves 12 aretriggered at staggered intervals, which will be explained with referenceto FIGS. 3 and 4.

In FIG. 3, the lines a, b, c, d each indicate signals for differentvalves 12. In the lines with the index 1 a respective synchronizingsignal 21, 22, 23, 24 generated by the control unit 18 is illustrated.The individual synchronizing signals 21-24 are staggered with respect toone another. Identical synchronizing signals, for example, the signal21, are generated at intervals with a period Tper. With four valves 12,different synchronizing signals are expediently staggered by a quarterperiod Tper/4 with respect to one another.

Further, in the lines that are provided with the index 2, control pulses25 for the valves 12 are illustrated. These control pulses 25 aregenerated by the respective controllers 13 for the associated valve 12,namely, in dependence on the temperature at the cooling surface 11determined by means of the temperature sensor 14. As long as the controlsignal 25 is present, that is, in the hatched time zones, the particularvalve 12 is open. In line a the open time is about 38% of the period. Inline b the opening degree is about 65% of the period. In line C theopening degree is about 55% of the period and in line d the open time isabout 14% of the period. The period can be, for example, five minutes.The percentage open time corresponds exactly to the mean opening degreeof the valve 12.

One can see that in this manner a relatively uniform loading of therefrigerant supply device can be effected. Certain fluctuations in therefrigerant requirement do occur, but these are restricted to relativelysmall portions of time. Such small portions of time can be catered forby the refrigerant supply in the collector 7.

Whereas in the embodiment of FIG. 3 a synchronizing pulse 21-24 isneeded for each valve, and has to be sent either by way of its own lineor with a corresponding address to the controller 13, in the embodimentaccording to FIG. 4 a single synchronizing pulse 26 is sufficient. Forthat purpose, all that is needed is for each controller 12 to beprovided with a delay element which triggers the control pulse 25 apredetermined duration after the start of the synchronizing pulse 26.The delay times Td are different for the four controllers. In this casethey change in increments of 1/4 period (Td=Tper/4). The pulse dutyfactor, that is, the ratio between the open time of the valve and theperiod Tper is similar to that in FIG. 3.

As one can see from FIG. 4, the synchronizing pulse 26 is supplied justonce by the control unit 18, for example, at the start of operation ofthe refrigeration system. Thereafter, each controller 13 is operatedautonomously. For that purpose it has another timer which providesclocked control of it at intervals corresponding to the period Tper.Each controller therefore has a periodic operation with the same period.Since inexpensive timers of acceptable accuracy are currently available,this option suffices to achieve a corresponding parallel operation ofthe controllers.

If instead of the illustrated four cooling surfaces a larger number ofcooling surfaces is used, for example, more than 10, the last coolingsurface may lag behind the first cooling surface by ten times the delaytime of the first valve. As a result, the behaviour of the refrigerationsystem could be too sluggish. In order to shorten the total responsetime, individual cooling surfaces can then be combined in groups, forexample, the first with the eleventh, twenty-first, thirty-first, thesecond with the twelfth, twenty-second, thirty-second etc.. The loadingfor the refrigerant supply device nevertheless remains relativelyuniform. The delay times remain long enough even with relatively shortperiods.

What is claimed is:
 1. Refrigeration system having a plurality ofcooling surfaces, each of which is connected by way of aseries-connected controlled valve to a common refrigerant supply device,and having a control arrangement connected to the valves, the controlarrangement having means for generating for each valve a pulse-widthmodulated signal for operation of the valve, all pulse-width modulatedsignals having a same period , and having means for opening the valvesof the individual cooling surfaces at staggered intervals with respectto one another.
 2. Refrigeration system according to claim 1, in whichthe period is constant.
 3. Refrigeration system according to claim 1, inwhich the valves are ON-OFF valves.
 4. Refrigeration system according toclaim 3, in which the control arrangement includes a controller for eachvalve, the controller having means to determine a pulse duty factor forthe valve in dependence on the demand for refrigeration and to supply acorresponding signal.
 5. Refrigeration system according to claim 4, inwhich the control arrangement includes means to produce a synchronizingpulse for each controller which is staggered with respect to thesynchronizing pulses for each other controller.
 6. Refrigeration systemaccording to claim 5, in which the control arrangement includes means togenerate a common synchronizing pulse for all controllers, eachcontroller having a time-delay element including a delay time whichdiffers from delay times for each other controller.
 7. Refrigerationsystem according to claim 5, in which each controller has a timer whichdetermines the period.
 8. Refrigeration system according to claim 7, inwhich the timers are activated by the synchronizing pulses responsive topredetermined operating conditions.
 9. Refrigeration system according toclaim 6, in which controller has a timer which determines the period.10. Refrigeration system according to claim 1, in which cooling surfacesare combined in groups.
 11. Refrigeration system according to claim 1,in which the refrigerant supply device includes a buffered supply. 12.Refrigeration system according to claim 11 in which the buffered supplycomprises a refrigerant collector.
 13. Refrigeration system according toclaim 1 including an overflow path for refrigerant supplied to thecooling surfaces.